Add ff_celp_circ_addf() function to be used for sparse vector circular

convolution in the upcoming AMR-NB floating point decoder. The function scales
and adds a vector, that is lagged by some offset, to another vector with the
same number of elements.

Patch by Colin McQuillan ( m.niloc googlemail com )

Originally committed as revision 19634 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/celp_filters.c

@@ -47,6 +47,16 @@ void ff_celp_convolve_circ(int16_t* fc_out,
     }
 }
 
+void ff_celp_circ_addf(float *out, const float *in,
+                       const float *lagged, int lag, float fac, int n)
+{
+    int k;
+    for (k = 0; k < lag; k++)
+        out[k] = in[k] + fac * lagged[n + k - lag];
+    for (; k < n; k++)
+        out[k] = in[k] + fac * lagged[    k - lag];
+}
+
 int ff_celp_lp_synthesis_filter(int16_t *out,
                                 const int16_t* filter_coeffs,
                                 const int16_t* in,